In general, in a wireless communication apparatus such as a radio receiver, an AGC (Automatic Gain Control) circuit is provided to adjust a gain of a reception signal. As a kind of the AGC circuit, there is an antenna damping circuit. The antennal damping circuit operates to reduce a level of a signal received by an antenna. When a reception intensity of an antenna input signal is not very large, the antenna damping circuit does not operate and a level of a reception signal is not reduced. However, when a signal in a strong electric field is inputted to the antenna, the antenna damping circuit operates to prevent excessive electric power from being applied to the wireless communication apparatus.
FIG. 1 is a diagram showing an example of a structure of a conventional antenna damping circuit. In FIG. 1, reference numeral 10 denotes a dummy antenna circuit. The dummy antenna circuit 10 is constituted by a resistor R1 and a capacitor C1 connected in series to a path extending from an input terminal Input to an output terminal Output of the antenna damping circuit and a capacitor C2 connected in parallel to the path.
Reference sign L1 denotes a coil for removing hum noise from a high-voltage wire; I1, a constant current source controlled according to a control voltage Vc; D1 and D2, PIN diodes; C3 and C4, capacitors; and R2, an output resistor. A not-shown RF (Radio Frequency) amplifier is connected to an output stage of this antenna damping circuit.
In the conventional antenna damping circuit constituted in this way, when a reception intensity of an antenna input signal inputted to the input terminal Input is not very large and the antenna input signal is extracted from the output terminal Output without being damped, the control voltage Vc is set to zero to prevent an electric current from flowing from the constant current source I1 to the PIN diodes D1 and D2. Consequently, the impedance of the PIN diodes D1 and D2 increases to be infinite and the antenna input signal, which has passed through the dummy antenna circuit 10, is directly transmitted to the output terminal Output without passing through a path of the PIN diodes D1 and D2. Thus, damping of the antenna input signal is not performed.
On the other hand, when a reception intensity of an antenna input signal inputted to the input terminal Input is large and the antenna input signal is damped and extracted from the output terminal Output, the control voltage Vc is applied to the constant current source I1 to supply an electric current from the constant current source I1 to the PIN diodes D1 and D2. Consequently, the impedance of the PIN diodes D1 and D2 decreases and the antenna input signal, which has passed through the dummy antenna circuit 10, flows to a ground potential point as well through the PIN diodes D1 and D2. The antenna input signal is substantially damped by the resistance of the PIN diodes D1 and D2 generated at this point and is transmitted to the output terminal Output.
Techniques for damping an antenna input signal using a PIN diode are also disclosed in, for example, Patent Documents 1 and 2.
[Patent Document 1]: Japanese Patent Application Laid-Open No. 5-121907
[Patent Document 2]: Japanese Patent Application Laid-Open No. 9-135180
As a general characteristic required of an antenna damping circuit, “a frequency characteristic of a damping amount in a necessary frequency band is flat”. In Patent Documents 1 and 2, contrivances are made to make a frequency characteristic of a damping amount flat.
In Patent Document 1, a variable capacitance element is connected in series or parallel to the PIN diode and a capacitance of the variable capacitance element is changed by a voltage for controlling a variable resistance of the PIN diode, whereby a frequency amplitude characteristic in a frequency band is made flat. In Patent Document 2, a high-pass filter including an antenna coupling capacitor and a resistor is formed and a frequency peak caused because of a resonant circuit, which is formed by a coil provided for measures against electric line interference in an antenna input circuit and another antenna capacitance element, is removed, whereby a frequency characteristic is made flat.
For example, in the case of an AM radio receiver, the dummy antenna circuit 10 including the capacitors C1 and C2 is provided as shown in FIG. 1. In the case of the antenna damping circuit including such a dummy antenna circuit 10, since the dummy antenna circuit 10 has a capacitive property, a frequency characteristic of a damping amount is not made flat being substantially affected by a capacitance of the dummy antenna circuit 10. In other words, a problem occurs in that, as long as a fixed electric current is fed to the PIN diodes D1 and D2, a damping amount in a reception frequency band of an AM broadcast is not fixed.
FIGS. 2A to 2C are graphs showing frequency characteristics of a damping amount in the conventional antenna damping circuit. As shown in FIG. 2A, when the control voltage Vc is set to zero to prevent an electric current from flowing from the constant current source I1 to the PIN diodes D1 and D2 (AGC is not applied), a level difference in the AM frequency band is within 1 dB and the damping amount has almost no frequency dependency. On the other hand, when the control voltage Vc is applied to feed electric currents of 100 μA and 1 mA from the constant current source I1 to the PIN diodes D1 and D2 (AGC is applied), as shown in FIGS. 2B and 2C, a level difference for each frequency increases in the AM frequency band and a damping error of about 10 dB at the maximum occurs.